Method and apparatus for transmission and reception within an OFDM communication system

ABSTRACT

A staggered spread OFCDM scheme is utilized that improves channel estimation. In a first embodiment, each chip stream is time shifted by a predetermined amount and then transmitted on a predetermined subcarrier. This results in time-spread symbols being staggered (time-offset) on different subcarriers allowing for more frequent sampling of the channel. In a second embodiment a staggered spreading approach is applied in the frequency dimension to improve the performance of a system with spreading in the frequency dimension.

FIELD OF THE INVENTION

The present invention relates generally to communication systems and inparticular, to a method and apparatus for transmission and receptionwithin a multicarrier communication system.

BACKGROUND OF THE INVENTION

Orthogonal Frequency Division Multiplexing (OFDM) is a well-knownmulticarrier modulation method that is used in several wireless systemstandards. Some of the systems using OFDM include 5 GHz high data ratewireless LANs (IEEE802.11a, HiperLan2, MMAC), digital audio and digitalvideo broadcast in Europe (DAB and DVB-T, respectively), and broadbandfixed wireless systems such as IEEE802.16a. An QFDM system divides theavailable bandwidth into very many narrow frequency bands (subcarriers),with data being transmitted in parallel on the subcarriers. Eachsubcarrier utilizes a different portion of the occupied frequency band.

Spreading can also be applied to the data in an OFDM system to providevarious forms of multicarrier spread spectrum. Such spread-OFDM systemsare generally referred to as either Spread OFDM (SOFDM), multicarrierCDMA (MC-CDMA), or Orthogonal Frequency Code Division Multiplexing(OFCDM). For systems employing MC-CDMA, spreading is applied in thefrequency dimension and multiple signals (users) can occupy the same setof subcarriers by using different spreading codes. For OFCDM, differentusers are assigned different mutually orthogonal spreading codes, andthe spread signals are combined prior to transmission on the downlink.Spreading can be applied in the frequency dimension, or the timedimension, or a combination of time and frequency spreading can be used.In any case, orthogonal codes such as Walsh codes are used for thespreading function, and multiple data symbols can be code multiplexedonto different Walsh codes (i.e., multi-code transmission).

Focusing on OFCDM systems, the orthogonality between Walsh codes is onlypreserved if the channel is constant over all of the time/frequencyresources that are spanned by the Walsh code. This leads to differenttradeoffs between time and frequency spreading for different systemparameters (e.g., subcarrier and OFDM symbol spacing) and differentchannel conditions (e.g., delay spread and Doppler spread).

For an OFCDM system with a spreading factor of SF in the time dimension,in which each symbol is represented by SF chips, up to SF Walsh codescan be active on each subcarrier. For channel estimation, one of theseWalsh codes can be assigned as a pilot signal (i.e., in the same waythat a pilot signal is created in conventional single-carrier CDMAsystems such as IS-95). However, a problem with this method is that whentime-variations are significant, for example due to vehicular mobility,the orthogonality of the Walsh codes is lost. This causes the pilotchannel to suffer interference from the other Walsh codes. Channelestimation is degraded due to this interference. Additionally, whendespreading the pilot channel, a single channel estimate results for theentire spread block of SF “chips.” This single channel estimate is notaccurate when the channel varies significantly over the block (SFchips). Therefore, a need exists for a method and apparatus fortransmission and reception within an OFDM communication system thatprovides a more accurate channel estimate, and reduces the amount ofpilot channel degradation for time-varying channels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 through FIG. 3 show examples of prior-art methods for includingpilot symbols in an OFDM-based system.

FIG. 4 illustrates a spread OFDM channel structure in accordance withthe preferred embodiment of the present invention.

FIG. 5 is a block diagram of a transmitter in a spread OFDMcommunication system in accordance with the preferred embodiment of thepresent invention.

FIG. 6 is a flow chart showing operation of the transmitter of FIG. 4 inaccordance with the preferred embodiment of the present invention.

FIG. 7 illustrates a spread OFDM channel structure in accordance withthe alternate embodiment of the present invention.

FIG. 8 is a block diagram of a transmitter in a spread OFDMcommunication system in accordance with an alternate embodiment of thepresent invention.

FIG. 9 is a flow chart showing operation of the transmitter of FIG. 7 inaccordance with the alternate embodiment of the present invention.

FIG. 10 illustrates channel estimation in accordance with the preferredembodiment of the present invention.

FIG. 11 is a block diagram of a receiver in accordance with thepreferred embodiment of the present invention.

FIG. 12 is a flow chart showing channel estimation in accordance withthe preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In order to address the above-mentioned need, a method and apparatus fortransmitting and receiving data in a spread OFDM system is providedherein. In particular, a staggered time-spread OFCDM scheme is utilizedthat improves channel estimation. In a first embodiment, each chipstream is time shifted by a predetermined amount and then transmitted ona predetermined subcarrier. This results in time-spread symbols beingstaggered (time-offset) on different subcarriers allowing for morefrequent sampling of the channel, improving channel estimation. In asecond embodiment a staggered spreading approach is applied in thefrequency dimension to improve the performance of a system withspreading in the frequency dimension.

The present invention encompasses a method for transmitting data in amulti-carrier system where data from an individual user is transmittedon multiple subcarriers. The method comprises the steps ofde-multiplexing a data stream to produce a plurality of de-multiplexeddata streams then spreading de-multiplexed data streams with a spreadingcode to produce a plurality of chip streams. Each chip stream is timeshifted by a predetermined amount and transmitted on a predeterminedsubcarrier.

The present invention additionally encompasses a method for transmittingdata. The method comprises the steps of de-multiplexing a symbol streamto produce a plurality of de-multiplexed symbols, and spreading eachsymbol with a spreading code to produce a plurality spread symbols, eachcomprising a predetermined number of chips. For a first transmissioninterval, a first chip of a spread symbol is mapped to a predeterminedsubcarrier and for a second transmission interval, the first chip of aspread symbol is mapped to a second subcarrier, wherein the secondsubcarrier differs from the first subcarrier.

The present invention additionally encompasses a method comprising thesteps of receiving a multicarrier signal comprising a plurality ofsubcarriers, demodulating the multicarrier signal to produce a chipstream, despreading the chip stream with a pilot code during a firstsymbol period to produce a first channel estimate for the first symbolperiod, despreading the chip stream with the pilot code during a secondsymbol period to produce a second channel estimate for the second symbolperiod, generating a third channel estimate only for a portion of thefirst symbol period based on the first and the second channel estimates,and generating a fourth channel estimate for a second portion of thefirst symbol period based on the first and the second channel estimates.

The present invention additionally encompasses an apparatus comprising ade-multiplexer, de-multiplexing a data stream to produce a plurality ofde-multiplexed data streams, a spreader spreading the de-multiplexeddata streams with a spreading code to produce a plurality of chipstreams, a time shifter, time shifting each chip stream by apredetermined amount, and a transmitter, transmitting each time-shiftedchip stream on a predetermined subcarrier.

The present invention additionally encompasses an apparatus comprising ade-multiplexer, de-multiplexing a symbol stream to produce a pluralityof de-multiplexed symbols, a spreader, spreading each symbol with aspreading code to produce a plurality spread symbols, each comprising apredetermined number of chips, and a mapper, for a first transmissioninterval, mapping a first chip of a spread symbol to a predeterminedsubcarrier and for a second transmission interval, mapping the firstchip of a spread symbol to a second subcarrier, wherein the secondsubcarrier differs from the first subcarrier.

The present invention additionally encompasses an apparatus comprising areceiver, receiving a multicarrier signal comprising a plurality ofsubcarriers and demodulating the multicarrier signal to produce a chipstream, a channel estimator, despreading the chip stream with a pilotcode during a first symbol period to produce a first channel estimatefor the first symbol period, and despreading the chip stream with thepilot code during a second symbol period to produce a second channelestimate for the second symbol period, and an interpolator generating athird channel estimate only for a portion of the first symbol periodbased on the first and the second channel estimates and generating afourth channel estimate for a second portion of the first symbol periodbased on the first and the second channel estimates.

Turning now to the drawings, wherein like numerals designate likecomponents, FIG. 1. and FIG. 2 show examples of prior-art methods forincluding pilot symbols in an OFDM-based system. Note that these priorart methods can be used for systems that transmit regular OFDM data, orspread data (such as MC-CDMA, OFCDM). However, note that each individualpilot symbol occupies only “one subcarrier by one OFDM symbol period”,and also note that the pilot and data are not code multiplexed. Instead,the pilot symbols separated in time and/or frequency from the data. Inthese prior art methods, a channel estimate may be obtained at eachpilot symbol location, which is separate from the data or spread datalocations. Then, the channel may be estimated at other locations in thetime-frequency grid, especially locations where data or spread data islocated, so the data can be despread and detected.

In contrast with the prior art methods of FIG. 1 and FIG. 2, thepreferred embodiment of the present invention comprises the use of aspread pilot that is code multiplexed with spread data.

FIG. 3 illustrates a prior art spread OFDM channel structure.Particularly, FIG. 3 illustrates an OFCDM system with spreading in thetime dimension. The time-frequency grid for this type of a system withSF=8 is shown where each symbol is spread with 8 chips. The eight chipsare then transmitted on a particular frequency (subcarrier). As shown inFIG. 3, eight chips representing a first symbol are transmitted onsubcarrier 1, followed by another eight chips representing anothersymbol. Similar transmissions occur on subcarriers 2 through 4. Up to SFsymbols can be code multiplexed onto the same time/frequency space. Forexample, up to SF symbols can be code multiplexed onto the samesubcarrier during a single spreading block interval, b. In a system witha code multiplexed pilot, at least one of the Walsh codes is used as apilot channel.

The composite signal at a particular location in the time-frequency gridis described as${x\left( {b,n,k} \right)} = {{c\left( {b,n,k} \right)}\left( {\underset{\underset{{pilot}\mspace{14mu}{channel}}{︸}}{{A_{p}\left( {b,k} \right)}{d_{p}\left( {b,k} \right)}{W_{p}\left( {n,k} \right)}} + \underset{\underset{{data}\mspace{14mu}{channels}}{︸}}{\sum\limits_{\underset{i = {1:{SF}}}{i \neq p}}{{A_{i}\left( {b,k} \right)}{d_{i}\left( {b,k} \right)}{W_{i}\left( {n,k} \right)}}}} \right)}$where:

-   b is the spreading block interval index (note that b increases by    one every SF OFDM symbol periods);-   n is the chip index within the b^(th) spreading block interval. Note    that n increments from 1 to SF within each spreading block interval    b;-   k is the subcarrier index, 1≦k≦K;-   c denotes the scrambling code;-   i is the Walsh code index, 1≦i≦SF;-   p denotes the Walsh code index that is used for the pilot channel;-   W_(i) denotes the i^(th) Walsh code;-   A_(i) denotes the (real) gain applied to the i^(th) Walsh code    channel (e.g., based on power control settings, if any); and-   d_(i) denotes the complex data symbol that modulates the i^(th)    Walsh code. d_(p) denotes the pilot symbol that modulates the p^(th)    Walsh code channel (i.e., the pilot channel).

Note that an OFCDM system has different characteristics thanconventional single-carrier CDM/CDMA systems. In single-carrier CDMAsystems, a common source of signal distortion is inter-chip interferencedue to multipath delay spread. This inter-chip interference destroys theorthogonality between different orthogonal spreading codes even thoughthe channel does not vary within a spreading block. The use of anOFDM-based multicarrier spread system such as OFCDM eliminates theinter-chip interference problem because of its reduced chip ratetogether with the cyclic prefix that is commonly used in OFDM-basedsystems. However, with the use of OFCDM, a new problem arises. In OFCDM,the chip duration is much greater than in a comparable-bandwidth singlecarrier system. As a result, the duration of a spreading block isgreatly expanded in an OFCDM system, and this creates an inherentproblem of sensitivity to channel variation over a spreading block.Channel variation within a spreading block causes interference betweenorthogonal spreading codes, and additionally leads to channel estimationproblems if a code multiplexed pilot is used.

As discussed above, prior art spread OFDM systems can lose orthogonalitywhen time-variations occur within the spread block. This causes thepilot channel to suffer interference from the other Walsh codes. Channelestimation is degraded due to this interference. Additionally, whendespreading the pilot channel, a single channel estimate results for theentire spread block of SF “chips.” This single channel estimate is notaccurate when the channel varies significantly over the block. In orderto address these issues, in the preferred embodiment of the presentinvention a staggered time-spread OFCDM scheme is utilized that improveschannel estimation. In particular, each chip stream is time shifted by apredetermined amount and then transmitted on a predetermined subcarrier.This results in time-spread symbols being staggered (time-offset) ondifferent subcarriers allowing for more frequent sampling of thechannel. Increased channel sampling rate results in improved channelestimator performance and improved channel tracking ability for higherdopplers (e.g., higher vehicle speeds or higher channel frequencies in amobile wireless system). Moreover, the present invention allows moreflexibility in selecting the parameters of an OFCDM system (such as SF,chip duration, number of subcarriers) since the resulting system is morerobust to channel variations.

FIG. 4 illustrates such a spread OFDM channel structure in accordancewith the preferred embodiment of the present invention. As is evident,from one subcarrier to another, the first chip for each symbol isstaggered in time. In this particular example, the “stagger offset” (SO)is equal to 4, so from one subcarrier to the next each symbol(comprising SF chips) is offset by 4 chip periods. For this example, aswith the example described in FIG. 3, SF=8, with each symbol beingspread with 8 chips. The eight chips are then transmitted on aparticular frequency (subcarrier). As shown in FIG. 4, sixteen chipsrepresenting up to SF*2 symbols are transmitted on subcarrier 1, withsixteen chips representing up to another SF*2 symbols being transmittedon subcarrier 2. However, the sixteen chips transmitted on subcarrier 2are time shifted so that transmission of the first chip takes placeduring the same time period as transmission of the 4^(th) chip onsubcarrier 1. A similar transmission pattern occurs for subcarriers 3and 4.

FIG. 5 is a block diagram of transmitter 300 in a spread OFDMcommunication system in accordance with the preferred embodiment of thepresent invention. As shown, transmitter 300 comprises de-multiplexer301, spreaders 302 and 304, time shifter 305, and OFDMmodulator/transmitter 306. For simplicity, data from a single user(e.g., uplink) or for a single user (e.g., downlink) is shown in FIG. 5,however one of ordinary skill in the art will recognize that in typicalOFCDM transmitters, multiple users transmit (or are transmitted to)simultaneously with up to SF symbols occupying the same time/frequencyspace. During operation a data stream from/for a user entersde-multiplexer 301 where the data stream is de-multiplexed into aplurality of data streams. Typical de-multiplexing operations convert adata stream at a given data rate (R) into N data streams each having adata rate of R/N.

Continuing, the de-multiplexed data streams enter spreader 302 wherestandard spreading occurs, producing a plurality of chip streams.Particularly, for an example scenario where the data and spreading codesare binary, spreader 302 modulo 2 adds an orthogonal code (e.g., an 8chip Walsh code) to data symbol. For example, in 8 chip spreading, datasymbols are each replaced by an 8 chip spreading code or its inverse,depending on whether the data symbol was a 0 or 1. More generally, thespreading code is modulated by a complex data symbol, for example d_(i)in the earlier equations; this complex data symbol may be selected froma M-ary QAM or M-ary PSK constellation, for example. The spreading codepreferably corresponds to a Walsh code from an 8 by 8 Hadamard matrixwherein a Walsh code is a single row or column of the matrix. Thus, foreach data stream, spreader 302 repetitively outputs a Walsh codemodulated by the present input data symbol value. It should be notedthat in alternate embodiments of the present invention additionalspreading or other operations may occur by spreader 302. For example,power control and/or data scrambling may be done, as shown in theprevious equation.

In the preferred embodiment of the present invention a single pilot persub-channel is broadcast along with each symbol stream, providingchannel estimation to aid in subsequent demodulation of a transmittedsignal. The single pilot channel is utilized by all users receiving dataduring the particular frequency/time period. In alternate embodiments ofthe present invention, the transmission of the pilot channel may be“skipped” at various time periods/subcarriers in order to transmit moredata when the channel conditions allow. A receiver, knowing the sequenceand time interval, utilizes this information in demodulating/decodingthe non-pilot broadcasts, which preferably occur on different spreadingcodes than the pilot. Thus in the preferred embodiment of the presentinvention a pilot stream (comprising a known symbol pattern) entersspreader 304, where it is appropriately spread utilizing a code from the8 orthogonal codes. The pilot chip stream is then summed with each datachip stream via summers 303. It should be noted that data for more thanone data stream may be summed at summers 303. In other words data foreach user transmitted during the particular frequency/time period willhave chips of multiple spreading codes summed at summers 303. Theresulting summed chip stream is output to time shifter 305.

As discussed above, time shifter 305 shifts specific chip streams on thedifferent subcarriers (frequencies) in time allowing for more frequentsampling of the channel. Particularly, adjacent channels have abeginning symbol period (e.g., beginning of each Walsh code) staggeredso that the beginning of one symbol period on a first subcarrier occursduring the transmission (preferably midway) of a second symbol period ona second subcarrier. All chip streams, whether time shifted or not, thenenter OFDM modulator 306 where standard OFDM modulation occurs.

FIG. 6 is a flow chart showing operation of the transmitter of FIG. 5 inaccordance with the preferred embodiment of the present invention. Thelogic flow begins at step 401 where a data stream from/for a user isde-multiplexed into a plurality of data streams. At step 403 each datastream is spread with a particular Walsh code and summed with a spreadpilot code (step 405). The summed chip streams enter time shifter 305where they are appropriately time shifted depending upon the subcarrierthey are to be transmitted on (step 407). Finally at step 409 OFDMmodulation and transmission occurs.

The above text described a system in which transmissions on differentsubcarriers were time shifted by a predetermined number of chips. Thisresults in time-spread symbols being staggered (time-offset) ondifferent subcarriers allowing for more frequent sampling of the channelin the time dimension, such that better estimates of the time-varyingchannel are obtained.

In an alternate embodiment of the present invention, spreading isperformed in the frequency dimension rather than (or in combinationwith) the time dimension. In this embodiment, channel variation occursover the subcarriers due to mutilpath delay spread, resulting in a lossof orthogonality between pilot and data spreading codes and difficultyin estimating the channel variations over the subcarriers. The staggeredspreading approach of the present invention is applied in the frequencydimension to improve the performance of a system with spreading in thefrequency dimension, as is shown in FIG. 7.

As shown in FIG. 7, during a first time period, a first chip of eachsymbol is transmitted on a first predetermined set of subcarriers(frequencies). During a second time period the first chip of each symbolis transmitted on a second predetermined set of subcarriers, where thesecond predetermined set of subcarriers differs from the firstpredetermined set of subcarriers. For a particular user, a first chip ofa spread symbol is mapped to a predetermined subcarrier during a firsttransmission interval, and then mapped to a second subcarrier during asecond transmission interval. In the preferred embodiment of the presentinvention the spread symbol is mapped to subcarriers k to K+SF−1 duringthe first transmission interval, and to m to m+SF−1 during the secondtransmission interval. Also note in FIG. 7 that multiple data symbol orspreading block periods can be represented in a single time interval(e.g., b=1 and b=2), since the chips of a spreading block do not need tospan multiple time periods with frequency-dimension spreading.

FIG. 8 is a block diagram of transmitter 600 in a spread OFDMcommunication system in accordance with an alternate embodiment of thepresent invention. As is evident, transmitter 600 is similar totransmitter 300 except that time shifter 305 has been replaced byfrequency/subcarrier mapper 605. Operation of transmitter 600 occurs asdescribed above with reference to FIG. 5 except that the summed chipstreams exiting summers 603 enter mapper 605 where they are mapped todifferent subcarriers as described above. In particular, for a firsttransmission interval, mapper 605 maps a first chip of a spread symbolto a predetermined subcarrier, and for a second transmission interval,mapper 605 maps the first chip of a spread symbol to a secondsubcarrier, wherein the second subcarrier differs from the firstsubcarrier.

It should noted that in both FIG. 4 and FIG. 7 there existsfrequency/chip locations that remain empty due to the staggering oftransmissions. These need not remain empty. For example, one could usethese spaces to transmit user data or control information (with orwithout a code multiplexed pilot) spread with a smaller spreadingfactor, or use a similar-length spreading factor and span multiple gapswith the user's data (with or without a code multiplexed pilot), or tosimply transmit additional pilot chips and/or pilot symbols that willfurther aid channel estimation at the receiver.

Additionally, variations of FIG. 4 and FIG. 7 in terms of the spreadingand the mapping of the spread symbols to the subcarrier/OFDM symbol gridare possible. In one alternate embodiment, the data symbols and pilotsymbol(s) may be spread with differing spreading factors, preferablybased on Orthogonal Variable Spreading Factor (OVSF) codes. For examplein FIG. 4 the pilot chip stream could have a spreading factor ofSF_pilot=8, while the data could have a spreading factor of SF_data=16.In this case a single spread data block of length 16 (as can be obtainedby concatenating two of the SF=8 spreading blocks such as b=1 and b=2 inFIG. 4) would contain two spread pilot symbols, each with SF_pilot=8,such that the receive processing for the pilot channel is substantiallysimilar to the preferred embodiment with reference to FIG. 4. Therefore,this embodiment provides additional flexibility in selecting or evendynamically adjusting the spreading factor used for data. However, forthis example note that the use of SF_pilot=8 blocks the use of 2 out ofthe 16 codes from the data channel, as is known in the art for OVSFcodes. In an additional example of this alternate embodiment, the spreaddata with a spreading factor of 16 can be mapped onto two differentsubcarriers to provide two-dimensional spreading on the data, which isknown in the art to provide additional frequency diversity. In thisexample, 8 chips of a 16 chip spreading block can be mapped tosubcarrier k=1 for spreading block interval b=1, and the remaining 8chips can be mapped to a different subcarrier (e.g., k=2 for spreadingblock interval b=1, k=3 for spreading block b=1 or b=2, or various otherpredetermined combinations).

FIG. 9 is a flow chart showing operation of the transmitter of FIG. 8 inaccordance with the alternate embodiment of the present invention. Thelogic flow begins at step 701 where a data stream from/for a user isdemultiplexed into a plurality of data streams. At step 703 each datastream is spread with a particular Walsh code and summed with a spreadpilot code (step 705). The summed chip streams enter frequency mapper605 where they are appropriately mapped to a particular subcarrier (step707). Finally at step 709 OFDM modulation and transmission occurs. Asdescribed above, during a first transmission period all symbols to betransmitted have their first chip transmitted on a first predeterminedset of subcarriers. During the next time period (chip period) allsymbols to be transmitted have their first chip transmitted on a secondpredetermined set of subcarriers. In the alternate embodiment of thepresent invention the first and the second set of subcarriers aremutually exclusive.

By utilizing the above described transmission schemes, a receiver isallowed more frequent sampling of the channel. During reception, abaseline channel estimate is preferably obtained per spreading block bydespreading the received signal by the pilot's Walsh code. The receivedsignal can be modeled as:r(b,n,k)=h(b,n,k)×(b,n,k)+η(b,n,k)where ^(h(b,n,k)) is the channel, and ^(η(b,n,k)) is thermal noiseand/or other noise and interference at the b^(th) block, n^(th) OFDMsymbol, k^(th) subcarrier. The pilot channel is preferably despread bymultiplying the received signal by the conjugate of the pilot's Walshcode times the scrambling code, and summing the elements; it is thenpreferably demodulated by dividing out the gain and pilot symbol:${\hat{h}\left( {b,k} \right)} = {{\frac{1}{A_{p}\left( {b,k} \right)} \cdot \frac{1}{d_{p}\left( {b,k} \right)} \cdot \frac{1}{SF}}{\sum\limits_{n = 1}^{SF}{{c^{*}\left( {b,n,k} \right)}{W_{p}^{*}\left( {b,n,k} \right)}{r\left( {b,n,k} \right)}}}}$

This despread channel estimate is the sum of three terms, one due to theconstant part of the channel, one due to thermal noise, and one due tointer-code interference (ICI) from the data users arising from channelvariation over the spreading block; in particular,${\hat{h}\left( {b,k} \right)} = {{\frac{1}{SF}{\sum\limits_{n = 1}^{SF}{h\left( {b,n,k} \right)}}} + {\eta^{\prime}\left( {b,k} \right)} + {\eta^{\prime\prime}\left( {b,k} \right)}}$where${\eta^{\prime}\left( {b,k} \right)} = {{\frac{1}{A_{p}\left( {b,k} \right)} \cdot \frac{1}{d_{p}\left( {b,k} \right)} \cdot \frac{1}{SF}}{\sum\limits_{n = 1}^{SF}{{c^{*}\left( {b,n,k} \right)}{W_{p}^{\prime}\left( {b,n,k} \right)}{\eta\left( {b,n,k} \right)}}}}$is the despread noise contribution and η″(b,k) is the term due to ICI

To improve the channel estimation, the baseline channel estimatesĥ(b,k), available once per spreading block and subcarrier, are combinedto take advantage of any correlation that exists across subcarriers, andto obtain per-chip channel estimates within the spreading block. Thefiltering and interpolation are now described. The combined channelestimate, ^(ĥ) ^(a,filt) ^((l,k)) is the final estimated channel at thek^(th) subcarrier of the l^(th) OFDM symbol, indexed by absolute symbolindex ^(l=1,2,3, . . .) . ^(ĥ) ^(a,filt) ^((l,k)) are obtained byinterpolating and filtering the spread block channel estimates,^(ĥ(b,k)), as detailed below. In one embodiment, the channel estimate isheld constant over the spread block and frequency filtering is applied.In another embodiment, the chip-level channel estimates are obtained byinterpolating the spread block channel estimates.

The channel estimates are first held constant for SO OFDM symbols, whereSO is the “stagger offset”, and the “stagger period” is defined${SP} \equiv {\frac{SF}{SO}.}$The special case of no staggering is obtained by setting SO=SF, andSP=1. The held channel estimates ^(ĥ) ^(a,held) ^((l,k)) indexed byabsolute time are “filled in” (i.e., sampled and held) with the despreadpilots, ĥ_(a, held)(l, k) = ĥ(b(l, k), k) where${b\left( {l,k} \right)} = {\left\lfloor \frac{l - 1 - {{SO} \cdot {{mod}\left( {{k - 1},{SP}} \right)}}}{SF} \right\rfloor + 1}$gives the block index for symbol l and subcarrier k. Note for a givenOFDM symbol l, different subcarriers come from possibly differentspreading blocks in the case of staggered spreading.

In the case of interpolation in the time dimension, for example linearinterpolation, the held channel estimates may be combined to obtainchannel estimates that vary with the chip index:${{\hat{h}}_{a,{lin}}\left( {l,k} \right)} = {\frac{1}{SF}{\sum\limits_{l_{1} = 1}^{SF}{{{\hat{h}}_{a,{held}}\left( {{1 + l_{1} - \frac{SF}{2}},k} \right)}.}}}$This procedure is illustrated in FIG. 10.

In the preferred embodiment of the present invention ^(ĥ) ^(a,held)^((l,k))(or ^(ĥ) ^(a,lin) ^((l,k)) for interpolated channel estimates)is then filtered across subcarriers for each OFDM symbol l. Thefiltering can be implemented in several ways. One way is to take anIFFT, and apply a multiplicative window to the time-domain channel tozero-out the portions corresponding to delay spreads larger than amaximum expected delay spread. Then the channel is obtained by taking anFFT. Another approach to the filtering is in the frequency domaindirectly. In either case, the channel is mathematically obtained viaapplying a low pass filter to all subcarriers,${{\hat{h}}_{a,{filt}}\left( {l,k} \right)} = {{\sum\limits_{k_{1} = 1}^{K}{{{\hat{h}}_{a,{held}}\left( {l,k} \right)}{g\left( {k_{1},k} \right)}\mspace{14mu}{or}\mspace{14mu}{{\hat{h}}_{a,{filt}}\left( {l,k} \right)}}} = {\sum\limits_{k_{1} = 1}^{K}{{{\hat{h}}_{a,{lin}}\left( {l,k} \right)}{g\left( {k_{1},k} \right)}}}}$where ^(g(k,k) ¹ ^(),1≦k) ¹ ^(≦K) are the channel estimation filtercoefficients for the k^(th) subcarrier. Note, some of the ^(g(k,k) ¹ ⁾may be zero.

The estimated channel at the n^(th) chip of the b^(th) spread block andk^(th) subcarrier is then given by ^(ĥ) ^(a,filt) ^((l,k)) at theappropriate time and frequency index; specifically, with no staggeringĥ(b,n,k)=ĥ _(a,filt)(l,k) with l=(b−1)·SF+n

For the case of staggered spreading blocks,ĥ(b,n,k)=ĥ _(a,filt)(l,k) with l=(b−1)·SF+n+SO·mod(k−1,SP)

The received signal is equalized, scrambling code removed and despreadto obtain an estimate of the transmitted data symbols,^({circumflex over (d)}) ^(i) ^((b,k)). Let ^(f(b,n,k)) be equalizercoefficient at the n^(th) chip of the b^(th) spread block and k^(th)subcarrier. The estimate of the transmitted data symbol modulated on thei^(th) Walsh code is then obtained by the following equation,${{\hat{d}}_{i}\left( {b,k} \right)} = {{\frac{1}{A_{i}\left( {b,k} \right)} \cdot \frac{1}{SF}}{\sum\limits_{n = 1}^{SF}{{c^{*}\left( {b,n,k} \right)}{W_{i}^{*}\left( {b,n,k} \right)}{f^{*}\left( {b,n,k} \right)}\; r\;\left( {b,n,k} \right)}}}$

The equalizer coefficient can be chosen according to different criteriasuch as EGC (Equal-gain chip combing) or MMSE criterion, $\begin{matrix}{{f\left( {b,n,k} \right)} = \frac{\hat{h}\left( {b,n,k} \right)}{{\hat{h}\left( {b,n,k} \right)}}} & {({EGC}),} \\{{f\left( {b,n,k} \right)} = \frac{\hat{h}\left( {b,n,k} \right)}{{{{\hat{h}}^{*}\left( {b,n,k} \right)}{\hat{h}\left( {b,n,k} \right)}} + {\sigma_{n}^{2}/\sigma_{x}^{2}}}} & {({MMSE}),}\end{matrix}$where σ_(n) ² is the variance of ^(η(b,n,k)) and σ_(x) ² is the varianceof x(b,n,k). If frequency-selective interference is present, then σ_(n)²/σ_(x) ² can be replaced with 1/SINR(b,n,k), where SINR is theSignal-to-Interference-plus-Noise Ratio. A gain correction term isfurther applied to the linear MMSE equalizer.

FIG. 11 is a block diagram of receiver 900 in accordance with thepreferred embodiment of the present invention. As shown, receiver 900comprises receiver/demodulator 901, buffer 902, despreader 903 channelestimator 904, chip-level interpolator 905, and multiplexer 906. Duringoperation, demodulator 901 receives multiple subcarriers (multicarriersignal) and demodulates them producing a plurality of chip streams. Thechip streams are passed to buffer 902 where they are stored. Buffer 902stores the demodulated chip stream for a predetermined time whilechannel estimation takes place. For each chip stream, channel estimator904 accesses buffer 902 and despreads the chip stream with a pilot codeduring a first symbol period (i.e., a first SF chips) to produce a firstchannel estimate for the first symbol period. In a similar manner,channel estimator 904 despreads the chip steam with the pilot codeduring a second symbol period to produce a second channel estimate forthe second symbol period. The channel estimates are passed to chip-levelinterpolator 905 where a third channel estimate is generated. Asdescribed above with reference to FIG. 11, the third channel estimate isgenerated only for a portion of the first symbol period (i.e., a portionless than SF chips), and is based on the first and the second channelestimates. In a similar manner, a fourth channel estimate is generatedof a second portion of the first symbol period based on the first andthe second channel estimates. The channel estimates are passed todespreader 903 where they are utilized in despreading the chip streamsinto multiple data streams. Multiplexer 906 then recombines the datastreams.

In summary, unlike prior-art channel estimation for multicarriersystems, in the preferred embodiment of the present invention per-chipchannel estimates are obtained from de-spread, code-multiplexed pilots,and these estimates can follow the channel variation within a singlespreading block even though the despread pilot provides only a singlechannel estimate per spreading block. As a result, each chip within asymbol potentially has a varying channel estimate, greatly improvingchannel tracking and despreader performance for higher dopplers, andenabling the use of a code multiplexed pilot for a larger range ofpotential system parameters.

FIG. 12 is a flow chart showing channel estimation in accordance withthe preferred embodiment of the present invention. The logic flow beginsat step 1001 where a multicarrier signal is received comprising aplurality of subcarriers. In the preferred embodiment of the presentinvention the received signal comprises a multicarrier signal havingrelatively time-shifted chip streams existing on at least twosubcarriers. The received signal is demodulated to produce a pluralityof chip streams (step 1003). At step 1005 the chip stream is despreadwith a pilot code during a first symbol period to produce a firstchannel estimate for the first symbol period, and at step 1007 the chipstream is despread with the pilot code during a second symbol period toproduce a second channel estimate for the second symbol period. In thepreferred embodiment of the present invention the first and the secondsymbol periods are non-overlapping in time and in the alternateembodiment of the present invention the first and the second symbolperiods are non-overlapping in frequency.

Continuing, at step 1009 a third channel estimate is produced for aportion of the first symbol period based on the first and the secondchannel estimates, and at step 1011 a fourth channel estimate isgenerated for a second portion of the first symbol period based on thefirst and the second channel estimates.

While the invention has been particularly shown and described withreference to a particular embodiment, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention.For example, although the above description was given primarilyinvolving OFDM modulation, one of ordinary skill in the art willrecognize that other multicarrier modulation techniques may be utilizedas well. Additionally, although the embodiments described above dealwith time and frequency spreading separately, one of ordinary skill inthe art will recognize that a combination of both simultaneous time andfrequency spreading as described above may be utilized as well. It isintended that such changes come within the scope of the followingclaims.

1. A method for transmitting data in a multi-carrier system where datafrom an individual user is transmitted on multiple subcarriers, themethod comprising the steps of: de-multiplexing a data stream to producea plurality of de-multiplexed data streams; spreading de-multiplexeddata streams with a spreading code to produce a plurality of chipstreams; spreading a pilot stream to produce a spread pilot stream;combining the spread pilot stream with a chip stream from the pluralityof chip streams; and time shifting each chip stream by a predeterminedamount; and transmitting each time-shifted chip stream on apredetermined subcarrier; wherein the step of time shifting each chipstream comprises the step of time shifting the combination of the pilotstream and the chip stream.
 2. The method of claim 1 wherein the step ofcombining the spread pilot stream with the chip stream comprises thestep of code multiplexing the spread pilot stream with the chip stream.3. The method of claim 1 wherein differing spreading codes are used forat least two of the de-multiplexed data streams.
 4. The method of claim1 further comprising the steps of: spreading a pilot stream to produce aspread pilot stream; adding the spread pilot stream with a data chipstream; time shifting the summed pilot stream by a predetermined amount;and transmitting the summed pilot stream and data chip stream on apredetermined subcarrier.
 5. A method for transmitting data, the methodcomprising the steps of: de-multiplexing a symbol stream to produce aplurality of de-multiplexed symbols; spreading each symbol with aspreading code to produce a plurality spread symbols, each comprising apredetermined number of chips; for a first transmission interval,mapping a first chip of a spread symbol to a predetermined subcarrier;and for a second transmission interval, mapping the first chip of aspread symbol to a second subcarrier, wherein the second subcarrierdiffers from the first subcarrier.
 6. The method of claim 5 furthercomprising the steps of: spreading a pilot stream to produce a spreadpilot stream comprising pilot chips; and combining the pilot chips withchips of the spread symbols such that the chips mapped to thesubcarriers comprise a combination of spread symbol chips and pilotchips.
 7. The method of claim 5 wherein the de-multiplexed symbolscomprises a code multiplexed pilot.
 8. The method of claim 5 furthercomprising the step of, for the first transmission interval, mapping thespread symbol to subcarriers k to k+SF−1, and for the secondtransmission interval, mapping the spread symbol to m to m+SF−1, whereinSF is a spreading factor and k does not equal m.
 9. A method comprisingthe steps of: receiving a multicarrier signal comprising a plurality ofsubcarriers; demodulating the multicarrier signal to produce a chipstream; despreading the chip stream with a pilot code during a firstsymbol period to produce a first channel estimate for the first symbolperiod; despreading the chip stream with the pilot code during a secondsymbol period to produce a second channel estimate for the second symbolperiod; generating a third channel estimate only for a portion of thefirst symbol period based on the first and the second channel estimates;and generating a fourth channel estimate for a second portion of thefirst symbol period based on the first and the second channel estimates.10. The method of claim 9 wherein the multicarrier signal furthercomprises a code multiplexed pilot.
 11. The method of claim 9 whereinthe step of receiving the multicarrier signal comprises the step ofreceiving a multicarrier signal having relatively time-shifted chipstreams existing on at least two subcarriers.
 12. The method of claim 9wherein the first and the second symbol period occur during a same timeperiod and comprise chips transmitted on differing subcarriers.
 13. Themethod of claim 9 wherein the first and the second symbol periods arenon-overlapping in time.
 14. The method of claim 9 wherein the first andthe second symbol periods are non-overlapping in frequency.
 15. Anapparatus comprising: a de-multiplexer, de-multiplexing a symbol streamto produce a plurality of de-multiplexed symbols; a spreader, spreadingeach symbol with a spreading code to produce a plurality spread symbols,each comprising a predetermined number of chips; and a mapper, for afirst transmission interval, mapping a first chip of a spread symbol toa predetermined subcarrier and for a second transmission interval,mapping the first chip of a spread symbol to a second subcarrier,wherein the second subcarrier differs from the first subcarrier.
 16. Anapparatus comprising: a receiver, receiving a multicarrier signalcomprising a plurality of subcarriers and demodulating the multicarriersignal to produce a chip stream; a channel estimator, despreading thechip stream with a pilot code during a first symbol period to produce afirst channel estimate for the first symbol period, and despreading thechip stream with the pilot code during a second symbol period to producea second channel estimate for the second symbol period; and aninterpolator generating a third channel estimate only for a portion ofthe first symbol period based on the first and the second channelestimates and generating a fourth channel estimate for a second portionof the first symbol period based on the first and the second channelestimates.
 17. The method of claim 1 wherein differing spreading codesare used for spreading the pilot and the de-multiplexed data stream.